1. Field of the Invention
This invention relates to monitoring changes in chemical levels and, more specifically, to methods and devices for determining such changes. By way of example, the invention may be used to monitor temporal changes of analyte levels in a source.
2. Description of the Related Art
Tracking of variable concentrations in, for example, fluids, is important for monitoring the status of physical, biochemical and biological systems. Such monitoring is useful to assess health, environmental conditions and medical concerns. Generally, it is desirable to monitor concentrations to identify trends indicative of certain conditions. For example, prompt awareness of an adverse condition is often essential to enable timely intervention. This is critically important when life and health are in jeopardy.
In the past, systems and methodologies for assessing chemical conditions, e.g., based on concentrations of representative species, have included conventional laboratory analyses and, to a lesser extent, portable assay methodologies. Assay methodologies are relatively complex and normally require analytical or quantitative procedures in order to observe features indicative of a desirable or undesirable condition. Accordingly, at least limited training is needed because the procedures entail, for example, mixing of fluids in predetermined proportions. Common examples of such procedures include analyses of water to determine whether concentrations of certain constituents, e.g., toxins in drinking water, have exceeded safe limits, or whether certain other constituents, e.g., swimming pool chemicals, have fallen below recommended levels.
While it is desirable to provide simplified assay procedures for a wide range of assessments, relatively few methods of performing an analysis have been found suitable for monitoring of chemical concentrations by non-specialists. It is especially desirable to provide simplified analysis procedures in the fields of environmental protection, health monitoring and medical diagnosis. The value of providing such procedures is evidenced by the disadvantages present in contemporary methodologies. Consider, for example, the medical field, wherein determination of chemical concentrations, such as hormone levels, may be indicative of disease or another medical concern. Typically, these types of quantitative assays require time-intensive laboratory analysis of blood or another body fluid.
A well-known case in point is the assessment required to confirm a healthy pregnancy. To effect that assessment, blood levels of chorionic gonadotrophin (commonly referred to as hCG) must be periodically monitored to confirm that hCG levels are rising rapidly during the first trimester of pregnancy. In particular, during the progression of a normal pregnancy, early hCG levels will typically double every two to three days. Studies show that among 85% of normal pregnancies such doubling occurs at least every 72 hours. Observation of this doubling is commonly used as an indication of a healthy pregnancy. On the other hand, in those circumstances when a consistent increase in the hCG level is not observed, there is a correlation with miscarriages or ectopic pregnancies.
A generally accepted method of monitoring temporal shifts in hCG levels requires a series of serum beta hCG quantitative tests. Typically, a Solid Phase Enzyme Linked Immunosorbent Assay (ELISA) is performed at designated time intervals to monitor changes in hCG concentration. Each ELISA test requires adding serum to an hCG antibody microtiter well. If hCG is present, it will bind to the antibody in the well. When another antibody, i.e., a label, is added it binds to the antibody-antigen complex. This labeling allows for development into a colored complex. This color is then analyzed spectrophotometrically such that the hCG concentration can be determined by the color intensity in order to indicate quantitative shifts in levels of hCG. The test is in common use because a measured decrease in the hCG level may be an early warning sign of a pregnancy complication.
While ELISA test results are highly accurate, the series of ELISA tests cannot be performed in a home environment or by an unskilled person. Rather, they require a laboratory analysis performed by trained technicians and, normally, a physician's approval. Further, the patient must make multiple visits to have the blood drawn. The results for each ELISA test will usually take at least 24 hours, necessitating an undesirable delay in obtaining important results. In addition, the time-consuming nature of the laboratory analysis renders this type of testing very expensive. Examples of ELISA tests for pregnancy are described in U.S. Pat. Nos. 5,198,366 and 5,182,216, each incorporated herein by reference.
Another example of periodic monitoring of an analyte level in a human sample is the routine screening for Prostate Specific Antigen, PSA. The level of PSA in a man's blood generally increases with age. However, a sudden or rapid rise, particularly in males with increased risk factors, may be an early warning sign of cancerous development within the prostate gland. Also, a slowly increasing level of PSA over time can also be indicative of potentially cancerous growth.
The American Cancer Society recommends that all men over the age of 50 be routinely screened by their doctor for any signs of prostate cancer. The PSA test is a blood test performed by a doctor that will give a quantitative determination of PSA level in the blood. A normal PSA level for the average man ranges from 0 to 4 ng/mL, while levels from 4 to 10 ng/mL are slightly elevated, and anything above 10 ng/mL can be considered abnormally elevated. During an annual physical, a man may opt to have the PSA test performed. If the results of the test show moderate levels, the man will generally not be retested further, at least until the next annual physical. The lengthy time period in between tests may be of concern for certain men with high risk factors. Additionally, in that time period, a sudden rapid rise may occur, but would not be apparent without more frequent testing.